Reproducing steps:
1. emacs -Q
2. Eval code: (add-to-list 'package-archives '("melpa" . "https://melpa.org/packages/“))
3. install the package iedit (version from melpa)
4. open the attachment c++ file, and goto line 262 and column 17. cursor will be on word “subsession"
5. M-x narrow-to-defun
6. M-x iedit-mode
7. M-x widen
8. You will see the lag when inputting chars.